This invention relates to an apparatus for detecting the stopping of a magnetic tape traveling in a magnetic recording and reproducing device such as a cassette tape recorder.
With a recording and reproducing device like a cassette tape recorder, a traveling tape is generally made to stop when the tape has run to its terminal end, and when a push button, for example, is operated to bring the tape to rest in a desired position. With the known recording and reproducing device, the mechanical process of detecting the tape stop in the case of stopping at the terminal end is carried out as follows. When a tape traveling between both tape reels with a proper tension has run to its terminal end, then the tension on the tape increases. This increased tape tension is transmitted to a takeup reel to actuate an electric switch, thereby detecting that the tape has traveled to its terminal end. However, the above-mentioned stop-detecting mechanical process has the drawbacks that a cassette tape recorder is complicated in construction, becomes bulky and has a lower durability. Recently, therefore, there has been proposed in place of the aforesaid mechanical process a system in which a pulse generator is provided to issue one pulse per rotation of a tape-driving rotor, thereby measuring a time interval between the respective pulses produced by said generator. With the proposed system, the above-mentioned time interval between the respective pulses is determined by a monostable multivibrator selected by a predetermined RC time constant. Therefore, said proposed system still has the drawbacks that the aforesaid step of driving the electric switch by the increased tension of a magnetic tape when it has run to its terminal end is carried out with a lower precision and further is unadapted for the ordinary or large scale integration (LSI) circuitry. In the case of operation using a push button, the push button signal is supplied as a direct stop signal to a built-in control circuit separately from the tape end detection signal used in the operation of stopping at the terminal end of the tape. Where, therefore, said control circuit is modified into an IC version, it is necessary to provide two input terminals one for the tape end detection signal and one for the push button signal. Such arrangement is not preferred for large scale integration (LSI) circuitry, which must contain as few terminals as possible.
With the proposed recording and reproducing device, a magnetic tape is made to run at widely different speeds for the respective functional modes. Where, under such condition, the tape stop is detected by measuring a time interval between pulses issued per rotation of the rotor, said detection is carried out by the RC time constant of a single monostable multivibrator. Therefore, it is necessary to set said RC time constant at a tape stop detection time corresponding to the longest among those used with the plural functional modes. Accordingly, the proposed recording and reproducing device has presented considerable difficulties in detecting the stop of a tape while running at different speeds by measuring a time interval between the respective pulses issued per rotation of the rotor.
Where the tape stop detection time is preset at the conceivably longest level, in other words, at a level required for the functional mode in which a magnetic tape is made to run most slowly, then as wide a difference as several seconds arises between a tape stop detection time during the feed mode in which a magnetic tape runs fastest and a tape stop detection time during the above-mentioned mode in which the magnetic tape travels most slowly, particularly with respect to a small battery-driven recording and reproducing device such as a microcassette tape. Where it is attempted to cut off power supply to, for example, a motor by a detection signal during the functional mode in which detection of a tape stop takes the longest time, then the motor consumes too large an amount of power to be overlooked during said period of several seconds. Consequently, it has been strongly demanded to improve the above-described pulse interval-measuring system for detection of a tape stop.